1. Field of the Invention
The present invention relates to appearance inspection apparatuses for inspecting defects in samples and more particularly relates to technology for inspecting the appearance of samples by use of scanning electron microscopes (SEM).
2. Description of the Related Art
Memories used in computers and the like, and semiconductor devices such as microcomputers are produced in the following manner: a pattern such as a circuit formed on a photo mask is repeatedly transferred by an exposure process, lithographic process, etching process, or the like. In semiconductor device manufacture, the results of lithographic, etching, or other processes, and defects caused by, for example, foreign matter significantly affect the manufacturing yield of semiconductor devices. Therefore, it is necessary to detect abnormalities or faults at an early stage or in advance. In order to detect these, patterns on semiconductor wafers are inspected at the end of each manufacturing process.
High throughput and highly accurate inspection are required to cope with larger diameter wafers and finer circuit pattern. This requires extremely high speed capturing of image data that has a high S/N ratio. For this purpose, a large electric current beam (100 nA or greater) 1000 times that of an ordinary scanning electron microscope (SEM) or greater is used, thereby obtaining the required number of electrons and maintaining a high S/N ratio. It is necessary to detect as speedily and efficiently as possible secondary or reflected electrons generated by a substrate.
Japanese Patent Application Laid-Open (JP-A) No. 2-142045 discloses the technology in which using an E×B filter, only a secondary electron beam is deflected toward a detector so as not to interfere with a primary electron beam.
In an inspection apparatus using such a scanning electron microscope (SEM), the quantity of electron beam emitted onto a sample is insufficient, resulting in inadequate contrast. To overcome such a problem, the time for which the electron beam is emitted is increased per pixel, and the sampling rate at which a digital luminance signal is generated from an analog luminance signal outputted from a detector is lowered below the standard value. Altering the sampling rate requires a process or device for altering a clock period.
JP-A No. 05-55919 discloses a method for operating a low resolution AD converter at high resolution. In this method, every predetermined number of successive sampling signals supplied from the AD converter are added and the result is then divided. This yields a digital signal with higher resolution.
According to the technology described in JP-A No. 05-55919, one pixel is created using each of the digital values of the final digital image signals obtained as the result of addition and division. The time for which an image is captured is obtained by multiplying the period of the final digital image signal by the number of pixels constituting one image screen. Accordingly, as the period of the final digital image signal is short, the time for which the image is captured is shortened. Accordingly, the throughput is improved.
However, if the period of the final digital image signal is shortened, scanning speed must also be increased, which would decrease the intensity of a luminance signal, and hence the S/N ratio. Any decrease in the S/N ratio degrades accuracy in defect detection.
If the period of the final digital image signal is lengthened, on the other hand, the scanning speed must be decreased, which would increase the strength of a luminance signal and hence the S/N ratio. The increase in S/N ratio improves accuracy in defect detection. However, the time for which an image is captured is lengthened and, accordingly, the throughput degrades.
That is, in the appearance inspection apparatus with the scanning electron microscope, the relation between defect detection performance and throughput is a trade-off. Generally, a user desires to give precedence to defect detection performance over throughput or vice versa, as necessity requires. However, conventional technology does not allow a user to give precedence to defect detection performance over throughput or vice versa as a user's desire.